1. Field of the Invention
The present invention relates generally to systems, apparatuses and methods for transporting and metering particulate material and, in preferred embodiments, to such a system, apparatus and method which employs multiple transport channels formed, in preferred embodiments, between three or more disks.
2. Description of Related Art
A wide variety of equipment has been used to either transport or meter particulate material (such as, but not limited to, coal, other mined materials, chemicals, dry food products, other dry goods handled in solid, particle form). Such transport equipment includes conveyor belts, rotary valves, lock hoppers, screw-type feeders, etc. Exemplary measurement or metering devices include weigh belts, gravametric and volumetric hoppers and the like. In order to provide both transport and metering of particulate material, it was typically necessary to use or combine both types of devices into a system.
However, applicant's earlier developments resulted in particulate transport devices which included the capability of both transporting and metering particulate material. Examples of such prior devices include the rotary disk type pumps discussed in the following U.S. patents, each of which is assigned or licensed to the assignee of present invention and each of which is incorporated herein by reference: U.S. Pat. No. 4,516,674 (issued May 14, 1985); U.S. Pat. No. 4,988,239 (issued Jan. 29, 1991); and U.S. Pat. No. 5,051,041 (issued Sep. 24, 1991); U.S. Pat. No. 5,355,993 (issued Oct. 28, 1994); U.S. Pat. No. 5,381,886 (issued Jan. 17, 1995); U.S. Pat. No. 5,485,909 (issued Jan. 23, 1996); U.S. Pat. No. 5,497,873 (issued Mar. 12, 1996); U.S. Pat. No. 5,551,553 (issued Sep. 3, 1996). Improvements, with respect to apparatuses described in the above-cited patents, for transporting and metering particulate material across a pressure differential (such as into a pressurized system environment) arc described in U.S. Pat. No. 5,402,976 (issued in Apr. 4, 1995), which is assigned to the assignee of the present invention and which is incorporated herein by reference.
According to the above-cited patents, particulate material may be transported and metered through a transport apparatus having a transport duct defined by at least one moving drive surface and a stationary surface. Embodiments illustrated in those patents include two moving surfaces defined by two rotary disks arranged coaxial and spaced from each other. An example of a two-disk apparatus as described in at least some of the above-cited patents is shown in prior art FIGS. 1 and 2 herein, wherein the apparatus 10 includes a housing 12 having an inlet 14, an outlet 16 and a drive rotor 18. The drive rotor 18 is composed of a hub 34 and a pair of disks 26 and 28. FIGS. 1 and 2 herein are substantially similar to FIGS. 1 and 2 of U.S. Pat. No. 5,402,876, cited above.
The drive rotor 18 (and, thus, the pair of disks 26 and 28) are mounted on a shaft 20, which is mounted for rotation relative to the housing and is coupled to a motor (not shown) for rotation in the direction of arrow 24. A transport channel is defined within the space between the opposing faces 36 and 38 of the disks 26 and 28, and is bounded on the inner and outer diameters of the disk faces by hub 34 and stationary inner walls 44 and 46 in the housing. As described in the above-cited patents, when the rotor 18 is driven in the direction of arrow 24, the disk faces 36 and 38 define moving drive surfaces which, in combination with the stationary surfaces of walls 44 and 46, act on the particles to cause the particles to inter-lock and bridge across the transport channel.
As the particles interlock with each other and bridge across the transport channel, the outermost particles engage the drive walls, such that drive force is transferred from the drive walls to the interlocked mass of particles. This interlocking and bridging action provides, in effect, a compacted transient solid spanning the width of the channel. Moreover, as a result of the drive force imparted by the drive walls, the transient solid of interlocked particles forms a moving dynamic mass, driven out of the outlet of the apparatus.
Two-disk transport apparatuses such as described above have proven to be capable of transporting and metering a variety of particulate materials that had previously been relatively difficult to efficiently transport and meter, including coal particles of varying sizes, grains having relatively low mass, sand, various chemicals, and chemical and mineral process feedstocks. Furthermore, because the moving dynamic mass of particulate material effectively fills the transport channel as it is moved out from the outlet of such apparatuses, the rate at which particulate material is transported out of the apparatus (the through-put of the apparatus) is a function of the cross-sectional area of the transport channel at the outlet and the rotation speed of the disks. Other factors, such as the bulk density of the material being transported also affect the transport rate. Thus, for a given material, channel cross-sectional area, and disk rotation speed, the feed rate of such an apparatus may be determined. In addition, feed rates of such apparatuses can typically be regulated, to some extent, by controlling and varying the disk rotation speed.
However, in various operational environments, the maximum speed by which the disks can be rotated may be limited, for example, by the capabilities of the available drive motor or transmission linkage, the type of material being transported, or other operational or environmental factors. Therefore, in such environments, greater transportation rates (through-put) traditionally required larger transport channels and, thus, larger disks. For example, disks as large as 60 inches in diameter have been used in two-disk transport apparatuses to meet feed rate requirements as high as 70 tons per hour, for a coal-fired power plant furnace.
The maximum size (diameter) of the disks may be limited by other operational and environmental constraints, for example, available space, weight handling capabilities, cost, safety or other factors. In addition, metering precision can be more difficult to control with a larger transport channel (disk diameter). Larger transport channels have a greater tendency to produce avalanching or cascading effects (surging and ebbing of the feed rate) at the outlet, instead of feeding out at a uniform feed rate.
An alternative to increasing the transport channel dimension (disk diameter) was noted by Donald Firth (inventor of the above-cited U.S. Pat. No. 4,988,239). More specifically, while preferred embodiments of the apparatus shown in the above-cited '239 patent employ a single drive rotor having two disks, Dr. Firth noted that "it is also possible to provide transport apparatus having multiple drive rotors which receive material from a single or multiple inlets" to provide "increased material through-put withou [sic] having to increase the diameter of the rotor disk" (column 7, lines 23-29 of the '239 patent). Similar statements are made in the above-cited '876 patent, at column 11, lines 36-42. With reference to FIG. 2 of the '239 patent, Dr. Firth illustrated a drive rotor composed of two disks (26 and 28), where each disk includes a section of the hub (34). Similarly, a drive rotor is illustrated in FIG. 2 of the '876 patent as having two disks separated by a hub.